1. Field of the Invention
The invention relates generally to a magnetic resonance method for forming a fast dynamic image and more particularly to a magnetic resonance method for forming a fast dynamic image from a plurality of signals acquired by an array of multiple sensors wherein prior to imaging, a sensitivity map of each of the sensors is provided, at least two adjacent sensors record signals originating from the same imaging position, which are multiplied by a sensitivity factor of the respective sensor at the respective imaging position while the image intensity is calculated from the signals measured by the various sensors, and the number of phase encoding steps is reduced from a full set. The invention also relates to a magnetic resonance imaging apparatus for obtaining a fast dynamic image from a plurality of signals, including an array of multiple sensors for recording signals, means for scanning the object along phase encoding trajectories, control means for skipping part of the phase encoding trajectories and calculating means for the reconstruction of an image from signals recorded by sensors in different spatial positions with respect to the object to be imaged in order to obtain a fast dynamic image, and to a computer program product stored on a computer usable medium for forming a fast dynamic image using the magnetic resonance method, comprising a computer readable program means for making the computer control the execution of a recording procedure for signals from an array of multiple sensors, a procedure for scanning the object along phase encoding trajectories, a control procedure for skipping part of the phase encoding trajectories, and a calculating procedure for the reconstruction of an image from signals recorded by sensors in different spatial positions with respect to the object to be imaged in order to obtain a fast dynamic image.
2. Description of Related Art
In magnetic resonance imaging there is a general tendency to acquire acceptable images in shorter periods of time. For this reason the Institute of Biomedical Engineering and Medical Informations, University and ETH Zxc3xcrich, Switzerland have recently developed an encoding method called xe2x80x9cSENSExe2x80x9d. The SENSE method is based on an algorithm which directly acts on the image as detected by the coils of the magnetic resonance apparatus; subsequent encoding steps can be skipped so that the signal acquisition for imaging can be accelerated by a factor two to three. Crucial for the SENSE method is the knowledge of the sensitivity of the coils which are arranged in so called sensitivity maps. In order to accelerate this method there are proposals to use raw sensitivity maps which can be obtained through division by either the xe2x80x9csum-of-squaresxe2x80x9d of the single coil references or by an optional body coil reference (see e.g. K. Pruessmann et. al. in Proc. ISMRM, 1998, abstracts p. 579, 799, 803 and 2087).
The SENSE method is preferred for acceleration of the signal acquisition for magnetic resonance imaging; this offers an enormous reduction in operating time. However, the method can only be used properly if the coil sensitivity is exactly known. Otherwise imperfections will cause fold-over artefacts which lead to incorrect images. In practice the coil sensitivity cannot be estimated perfectly and will be dependent on fluctuations in time (movement of the patient, temperature influences, etc.).
It is an object of the present invention to reduce to a great extent or even eliminate the fold-over artefacts during fast dynamic imaging.
This and other objects of the invention are achieved by a method in which prior to the formation of the fast dynamic image, a normal magnetic resonance image with the full set of phase encoding steps is acquired for each sensor, a subset of phase encoding trajectories is extracted commensurate with the phase encoding trajectories obtained by the fast dynamic imaging and an image is reconstructed from the subset of phase encoding trajectories, and the signals of the fast dynamic image and the signals of the reconstructed image are compared in order to obtain an estimate of the fold-over artefacts of the fast dynamic image such that the signals of the fold-over artefacts are compensated in the signals obtained by the fast dynamic imaging. This and other objects of the invention are also achieved by an apparatus including means for signal acquisition for a normal magnetic resonance image with a full set of phase encoding steps whereby a subset of phase encoding trajectories is extracted commensurate with the phase encoding trajectories obtained by fast dynamic imaging and an image reconstructed from the above-mentioned subset, calculating means for comparing the signals of the fast dynamic image with the signals of the reconstructed image, thus yielding an estimate of the fold-over artefacts of the fast dynamic image, and compensating means for compensating the signals of the fold-over artefacts in the signals obtained by the fast dynamic imaging. This and other objects of the invention are also achieved by a computer program stored on a computer usable medium for forming a fast dynamic image using a magnetic resonance method comprising a computer readable program means for making the computer control the execution of a signal acquisition procedure for a normal magnetic resonance image with a full set of phase encoding steps whereby a subset of phase encoding trajectories is extracted commensurate with the phase encoding trajectories obtained by fast dynamic imaging and an image reconstructed from the above-mentioned subset, a calculating procedure for comparing the signals of the fast dynamic image with the signals of the reconstructed image, thus yielding an estimate of the fold-over artefacts of the fast dynamic image, and a compensating procedure for compensating the signals of the fold-over artefacts in the signals obtained by the fast dynamic imaging.
The main issue of the present invention is based on the supposition that the shape of the fold-over artefacts varies quite slowly in time, i.e. the previous imperfections from the estimate of the profile of the coil sensitivity will also vary only slowly with respect to the imaging time. The idea of the measuring method is in fact such that an estimate of the artefacts will be acquired, which estimated artefact image may be subtracted from the image provided by fast dynamic imaging. This means that the aforementioned SENSE method can be accelerated since a calibration of the coils in the sense of tedious measuring of sensitivity maps will no longer be necessary.
A further advantage of the method according to the present invention consists in that the estimate of the artefact image is continuously corrected; that is a more precise estimate of the image artefact is obtained iteratively, thus enhancing the quality and reliability of the fast dynamic imaging method. Since the signal-to-noise ratio of the fast dynamic image and that of the artefact image are of the same order of magnitude the signal-to-noise ratio of every corrected fast image will be increased. Namely, the corrected fast image is formed by superposition of the fast dynamic image and the artefact image; hence, if no steps are taken the noise contribution of the artefact image and the fast dynamic image will add vectorially. In order to overcome this problem, another acquisition scheme can be used in that the sampling of sets of profiles during fast dynamic imaging is shifted on step in k-space. This will improve the signal-to-noise ratio, thus yielding a more reliable image.